What about Neil Young and Greg Allman ( Mr Cher).
Yeh Neil Young is a liberal piss off however he does have some good songs along the way.
Neil Young rates high for me as well.
I first thought the picture of Bob Dylan was of a withered Vincent Price.
In all fairness, the criticism of Frank Zappa could be best described as his not being mainstream at all. He should not have been thought of as a rock’n’roller, because that is not what he was about. He created music for musicians. This included chamber music:
http://www.nytimes.com/2007/02/05/arts/music/05zapp.html